XGMAC supports maximum MTU that can go to 16KB. Lets add this check in
the calculation of RX buffer size.

Fixes: 7ac6653a ("stmmac: Move the STMicroelectronics driver")
Signed-off-by: NJose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>